From U.S. Pat. Nos. 3,244,377 and 4,266,729 it is known to dispose a plastic coating or sleeve over the outside of a metal valve body of a fuel injector. The former patent discloses a tetrafluoroethylene coating applied by spraying. A stated purpose of the coating is to prevent misalignment caused by removal of the nozzle from the cylinder head bore. It is also said to resist adherence of carbonaceous exhaust products. The latter patent discloses a protective sleeve made of plastic or a heat-shielding material and press-fit onto the body. The O.D. of the body has a shallow circular groove that is covered by the sleeve.
While the present invention relates to the disposition of a poorly thermally conductive sleeve on the outside of a metal valve body of a fuel injector in covering relation to a groove that extends circumferentially around the O.D. of the valve body, principles of the invention are conceptually different from the disclosures of the above-discussed patents.
The present invention relates to improving the so-called "hot fuel handling" performance of a top-feed fuel injector for a liquid-fueled, spark-ignited internal combustion engine. A known top-feed fuel injector comprises a metal valve body that is shaped to fit a certain size receptacle when installed on an engine. It has been observed that this metal body has a mass which possesses a certain thermal capacitance. Under certain conditions, this thermal capacitance delivers thermal energy to liquid fuel within the fuel injector. By limiting thermal energy transfer to liquid fuel within the fuel injector, the risk that the fuel will change phase before it is injected from the fuel injector is also limited. Vaporization of liquid fuel within the fuel injector is undesirable because it impairs the metering accuracy of an injection pulse.
According to principles of the invention, the thermal capacitance of the metal valve body is reduced by a selective reduction in the thickness of its sidewall, accompanied by the use of a poor thermal conductor in replacement of the removed material so that the shape and displacement volume of the resulting composite valve body remains the same as in the one-piece all metal valve body. Importantly, the replacement material does not merely fill the void created by the removed metal. Rather, the replacement material is a sleeve that in cooperation with the reduced thickness sidewall of the metal valve body defines a totally enclosed, poorly thermally conductive space extending circumferentially around the composite valve body. A suitable material for the sleeve is a dimensionally stable nylon that possesses acceptable characteristics for automobile engine applications, and the enclosed space is a dead air space that is an even better (perhaps as much as seven or eight times better) thermal insulator than nylon. The creation of a dead air space lowers the thermal capacitance of the composite valve body even more than replacing the entirety of the removed metal with all nylon, and this means that less nylon has to be used. Accordingly, the incorporation of a dead air space offers distinct and significant advantages. The composite valve body is readily fabricated by pressing a suitably shaped nylon sleeve over an all-metal body.